This invention relates to intraocular lenses. In particular, the present invention relates to toric intraocular lenses.
Foldable intraocular lens (xe2x80x9cIOLxe2x80x9d) materials can generally be divided into three categories: silicone materials, hydrogel materials, and non-hydrogel acrylic materials. Many materials in each category are known. See, for example, Foldable Intraocular Lenses, Ed. Martin et al., Slack Incorporated, Thorofare, N.J. (1993). Biocompatibility varies among different IOL materials within and among each category.
One measure of biocompatibility for an IOL can be the incidence of posterior capsule opacification (xe2x80x9cPCOxe2x80x9d). A number or factors may be involved in causing and/or controlling PCO. For example, the design and edge sharpness of an IOL may be a factor. See, Nagamoto et al., J. Cataract Refract. Surg., 23:866-872 (1997); and Nagata et al., Jpn. J. Ophthalmol., 40:397-403 (1996). See, also, U.S. Pat. Nos. 5,549,670 and 5,693,094. Another factor appears to be the lens material itself. See, for example, Mandle, xe2x80x9cAcrylic lenses cause less posterior capsule opacification than PMMA, silicone IOLs,xe2x80x9d Ocular Surgery News, Vol. 14. No. 15 (1996). See, also, Oshika, et al., xe2x80x9cTwo Year Clinical Study of a Soft Acrylic Intraocular Lens,xe2x80x9d J. Cataract. Refract. Surg., 22:104-109 (1996); and Ursell et al., xe2x80x9cRelationship Between Intraocular Lens Biomaterials and Posterior Capsule Opacification,xe2x80x9d J. Cataract Refract. Surg., 24:352-360 (1998).
One method of addressing the PCO problem involves administering a pharmaceutical agent to the capsular bag area at the time of, or immediately after, extracapsular cataract extraction. See, for example, U.S. Pat. Nos. 5,576,345 (pharmaceutical agent=the cytotoxic agent taxol or an ophthalmically acceptable derivative); 4,515,794; and 5,370,687. Alternatively, the pharmaceutical agent may be tethered to the surface of the IOL material. See, for example, U.S. Pat. No. 4,918,165. The pharmaceutical agents are intended to kill or prevent the growth of proliferating cells that might cause PCO or xe2x80x9csecondary cataracts.xe2x80x9d Yet another method involves the physical destruction or removal of lens epithelial cells. See, Saika et al., J. Cataract Refract. Surg., 23:1528-1531 (1997).
Another method of addressing PCO is the prophylactic laser therapy method disclosed in U.S. Pat. No. 5,733,276. According to this method, the lens capsule is irradiated with laser irradiation to destroy cells that remain in the lens capsule after extraction of a cataract.
Other methods theorized for reducing the risk of PCO involve adhering the posterior capsule to the IOL at the time of implantation, as in U.S. Pat. No. 5,002,571. According to the ""571 patent, a non-biological glue or, preferably, a biological glue, such as fibrin, collagen, or mussel glue, is used to adhere the posterior lens capsule to the posterior surface of an IOL. The glue may be applied over the entire posterior surface of the IOL or just as an annulus around the outer perimeter of the posterior surface of the IOL.
In contrast, U.S. Pat. No. 5,375,611 discloses a method of reducing the risk of PCO by preventing the adherence of the posterior capsule to the IOL. According to the ""611 patent, the posterior surface of the lens capsule itself is chemically modified at the time of extracapsular cataract extraction. The chemical modification is achieved by depositing a water-insoluble stable or permanent layer of a cell attachment-preventing compound onto the posterior surface of the lens capsule. The stable or permanent layer may be a polymer, such as polyethylene glycol, polysaccharides, polyethylenepropylene glycol, and polyvinyl alcohol.
Aside from biocompatibility concerns, positional stability after implantation is a very important concern for toric IOLs. Toric IOLs are designed to be oriented in a specific way in order to provide the desired vision correction. These IOLs should not rotate or slip from their implanted position.
The present invention relates to a method of selecting an IOL material for reducing the risk of posterior capsule opacification. IOL materials having a certain tack are more likely to reduce the risk of posterior capsule opacification than are materials having a lower tack. Tack is determined by measuring the maximum load required to separate two pieces of the same material.
The present invention also relates to a method of selecting a material for toric IOLs. IOL materials having a certain tack allow implanted toric IOLs to remain in their intended position and provide their designed correction.